The present application claims priority to Japanese Application No. P2000-022765 filed Jan. 31, 2000, which application is incorporated herein by reference to the extent permitted by law.
1. Field of the Invention
The present invention relates to an optical recording medium.
2. Description of the Related Art
In recent years, optical recording mediums have received a remarkable attention as high-density recording mediums. Of conventional optical recording mediums, a magnetooptical recording medium, for example, includes a transparent substrate made of a suitable material such as polycarbonate on which a dielectric layer, a recording layer formed of a magnetic layer, a dielectric layer and a reflection layer made of a metal are laminated, in that order, and in which signals are recorded on/reproduced from the magnetooptical recording medium with irradiation of laser light from the transparent substrate side.
In this optical recording medium, in order to increase its recording density, it is intended to reduce a diameter of a spot of laser light by using short-wavelength laser light (blue laser light) or by using an optical system having a large numerical aperture (NA). In this case, an energy density of a spot of laser light on a recording layer increases considerably and a Kerr rotation angle xcex8k obtained upon reproduction decreases. Then, there arises a problem that a signal characteristic will be deteriorated. The reason for this will be described below.
That is, when the recording layer is irradiated with laser light of high energy density, a temperature on the recording layer increases and the Kerr rotation angle (k decreases. When a temperature on the recording layer increases more, a magnetic recording on the recording layer is lost so that the signal characteristic is deteriorated.
It has been reported that the above problem could be solved by directly depositing a heat control layer made of Ag, Al or the like on the recording layer at its surface opposite to the substrate to suppress a temperature rise on the recording layer (see U.S. patent application Ser. No.09/496,795).
In an ordinary magnetooptical recording medium which is recorded/reproduced with irradiation of laser light from the substrate side, the arrangement of layers is such one in which a substrate, a dielectric layer (SiN, etc.), a recording layer (TbFeCo, etc.), a heat control layer (Ag, Al, etc.), a dielectric layer (SiN, etc.) and a metal reflection layer (Al, etc.) are laminated, in that order. Specifically, since the heat control layer is laminated on the recording layer after the recording layer had been deposited on the dielectric layer, the heat control layer can be prevented from affecting the underlayer of the recording layer.
On the other hand, the following magnetooptical recording medium has been developed so far in order to effect a higher-density recording.
This magnetooptical recording medium is a so-called surface reading magnetooptical recording medium comprising a substrate on which a reflection layer, a dielectric layer, a recording layer, a dielectric layer and an organic protection layer are laminated, in that order, and in which signals are recorded/reproduced by irradiating and focusing laser light on the recording layer from the organic protection layer side opposite to the substrate through a lens system so that a space between the lens system and the recording layer can be reduced.
However, when the heat control layer is formed on this surface reading magnetooptical recording medium, the arrangement of layers is presented as shown in FIG. 1 in such a manner that a substrate 2, a reflection layer 6 (Ag alloy, etc.), a dielectric layer 3 (SiN, etc.), a heat control layer 1 (Ag, Al, etc.), a recording layer 4 (TbFeCo, etc.) and a dielectric layer 5 (SiN) are laminated, in that order. Consequently, the heat control layer 1 is deposited as the underlayer of the recording layer 4 and its surface roughness exerts an influence upon a disk noise. If the surface of the heat control layer 1 had been smoothed by reverse sputtering after the heat control layer such as Ag or Al was deposited, a satisfactory smooth surface would not be obtained.
When the heat control layer 1 is deposited on the recording layer 4 at its one surface opposing the substrate 2 side, the recording layer 4 is deposited on the heat control layer 1 after the heat control layer 1 was deposited on the dielectric layer 3 so that the heat control layer 1 is unavoidably deposited as the underlayer of the recording layer 4. As a result, if Ag, Al or the like is used as the material of the heat control layer 1, there then arises a problem that a disk noise such as an erase noise will increase due to the surface roughness of the heat control layer 1.
The above erase noise is such one generated when the magnetooptical recording medium is reproduced after a magnetization direction of the recording layer had been oriented in one direction with application of a magnetic field to one direction from the outside or such one generated when the magnetooptical recording medium is reproduced after the direction of the magnetic field applied from the outside had been reversed to that of the recording mode and laser light had been focused and irradiated on the magnetooptical recording medium or such one generated when the magnetooptical recording medium is reproduced after the direction of the magnetic field applied from the outside had been reversed to that of the recording mode and the recorded signal had been erased by focusing and irradiating laser light on the magnetooptical recording medium.
In view of the aforesaid aspect, it is an object of the present invention to provide an optical recording medium in which a heat characteristic can be improved and a disk noise can be reduced.
According to an aspect of the present invention, there is provided an optical recording medium in which a reflection layer is deposited on at least one substrate, a dielectric layer is deposited on the reflection layer, a heat control layer is deposited on the dielectric layer and a recording layer is directly deposited on the heat control layer and in which information is at least recorded or reproduced with irradiation of laser light from the opposite side of the substrate and in which the heat control layer is comprised of an AgPdCu alloy thin layer.
In the optical recording medium according to the present invention, a surface roughness Ra of the above heat control layer is selected to be less than 0.75 nm.
Further, in the optical recording medium according to the present invention, the surface roughness Ra of the above heat control layer is selected to be less than 0.75 nm and a surface roughness Ra of the reflection layer is selected to be less than 0.75 nm.
Furthermore, in the optical recording medium according to the present invention, the reflection layer is comprised of an AgPdCu alloy thin layer. Alternatively, the reflection layer is comprised of an AgPdCuAl alloy thin layer.
According to the present invention, there is obtained an optical recording medium in which the recording layer is directly formed on the heat control layer and in which information is recorded or reproduced with irradiation of laser light from the opposite side of the substrate. Since the heat control layer is comprised of the AgPdCu alloy thin layer, there can be obtained a heat control layer having a smooth surface as well as a high thermal conductivity.